The invention relates to self-cleaning centrifugal separators having a main piston valve defining one side of the separating chamber and connected with at least one auxiliary piston valve, the main piston valve opening and closing the sludge ejection apertures in the periphery of the drum and the auxiliary piston valve connected with the main piston valve additionally influencing the opening action of the main piston valve. A separator of this kind is known, for example, through French Pat. No. 963,524.
Separators of this type of construction have only one control chamber located on the side of the main piston valve that is remote from the separating chamber. When control fluid is admitted into this chamber, the fluid pressure that builds up and acts upon the outer side of the main piston valve moves the latter to the closed position. When the control fluid is let out of this chamber, the pressure of the drum charge acting on the inner side moves the main piston valve to its open position. The auxiliary piston valve fixedly joined by bolts to the main piston valve produces, by means of the control fluid, an additional opening force which is transmitted to the main piston valve.
This additional opening force is necessary in types of separators in which the main piston valve defines one side of the separating chamber, because the pressure acting to open the main piston valve in the event, for example, of a complete emptying, diminishes as the level of the drum charge drops, so that the main piston valve does not reach its full open position and thus also does not open the annular gap to its full width. Consequently, the uniform ejection of solids which, for example, tend to cake up or mat together is not assured. Solids which remain unequally distributed in the separator, however, can result in severe imbalance of the separator drum.
Another type of construction is known through German Pat. No. 2,041,371 (U.S. Pat. No. 3,754,701), in which the main piston valve defining one side of the separating chamber is provided with a cap over its central portion, which is carried on an axial projection of the piston valve with the use of a sealing ring sealing it off from the separating chamber, and forms with the main piston valve a chamber which can be charged with control fluid. Since this chamber, which is always filled with control fluid, produces a constant opening force, the annular gap is opened to its full width by the main piston valve for the uniform ejection of the solids even as the level of the drum charge recedes.
In the centrifugal separation of specifically heavier liquids and suspended solids which are of even greater specific weight, it can happen that the ratio between the closing force and the opening force--especially in the case of partial emptying where the full opening force is still largely sustained--is not great enough, and the main piston valve therefore does not bring the drum into the closed position rapidly enough or fails to close it tightly enough, i.e., the closing force is too weak.
Both of the above-described types of construction--the one based on French Pat. No. 963,524 and the one based on German Pat. No. 2,041,371--will then fail to operate satisfactorily, since an additional opening force is provided, but no additional closing force. Furthermore, the chamber of the type of construction described in the German patent is located above the piston valve within the separating chamber, and is separated from the separating chamber, and thus from the material being separated, only by a sealing ring. If this ring, which is subjected to severe wear in the emptying actions, develops a leak, either control fluid will get into the separating chamber and mix with the material being centrifuged, or the material being centrifuged will get into this chamber, and both of these circumstances must be avoided.
In the centrifugal separation of fermentation liquids, bacterial cultures, liquid foods and the like, infections would immediately occur if control fluid should come in contact with the material being centrifuged.
An improvement over German Pat. No. 2,041,371 is described in German Pat. No. 2,263,664 (U.S. Pat. No. 3,905,292). In this improved design, the main piston valve surface that is acted upon to open the drum can be made by far smaller than in previously known centrifugal separators, so that the closing force will predominate sufficiently even in the case of liquids of substantially greater specific weight. The hydraulic chamber, which is provided in this design too, is nevertheless also located in the centrifugal separating chamber, with the same disadvantages mentioned before, and the arrangement of the chamber within the separating chamber additionally has a negative effect on the disk stack area of the drum and hence on the degree of separation.